1. Field of the Invention
The present invention relates to a wavelength-division-multiplexing optical amplifier device for amplifying a plurality of optical signals having different wavelengths, and more particularly to the stabilization of an amplified optical output signal from such a wavelength-division-multiplexing amplifier device. The present invention is also concerned with a wavelength-division-multiplexing optical transmission system which employs wavelength-division-multiplexing optical amplifier device in an optical transmitter and an optical repeater.
2. Description of the Related Art
Conventional optical linear amplifier devices are controlled by an optical amplifier control process which branches an inputted optical signal of the optical linear amplifier, detects the reception level of the optical signal with a reception level monitor, and controls the amplification factor of the optical linear amplifier depending on the detected reception level. However, no reliable control system has been established yet for an optical transmission system where a plurality of different optical signals are wavelength-division-multiplexed for transmission, particularly an optical linear amplifier that employs optical pumping for its operation.
Wavelength-division-multiplexing optical transmission systems to which conventional optical linear amplifiers are directly applied suffer various problems. In such a wavelength-division-multiplexing optical transmission system, two or more optical signals having different wavelengths are wavelength-division-multiplexed in a single wavelength band, and amplified by an optical amplifier for transmission. One of the problems is that when one or more optical signals are interrupted for some reason, such a signal interruption is erroneously detected as a reduction in the inputted level of the combined optical signal, and the system increases the amplification factor of the optical amplifier, resulting in an increase in the amplification factor for each of the optical signals. This phenomenon happens because the conventional optical amplifier control process branches an inputted optical signal, detects the reception level of the optical signal with a reception level monitor, and controls the amplification factor of the optical amplifier depending on the detected reception level.
Another problem is that while the amplification factor for each of the optical signals may be set to a desired level at an upstream optical signal terminal of the wavelength-division-multiplexing optical transmission system, the amplification factor for each of the optical signals tends to vary depending on the number of optical signals at a downstream linear repeater. This is because there has been no means whatsoever for transmitting information representative of the number of optical signals from the upstream optical signal terminals to the downstream linear repeater.